Boring cylinders on a small bench lathe.

Hi all,

I'm wondering how best to go about boring a cylinder on a small bench lathe, at the moment I only have this small budget boring bar which I bought to get me started in this hobby.

It's an 8mm shank and when I bought it I seem to remember the maker stated it was designed for a max depth of 38mm (although I can no longer find that info so could be mistaken). It's been fine for the small jobs I've needed it for but I'm currently making a small horizontal engine that needs a 45mm deep bore and I'm also trying to make a 'beefy' wobbler a bit bigger than the usual 10 -15mm bore just as a bit of fun.

Presumably the bar I have will be inadequate ?

Can any one direct me to the sort of bar I might need or how I might go about making a bore of perhaps 60mm long on a 8 x 16 lathe ?.

To be honest as a beginner the range of bars and the vast difference in price is a bit bewildering and I don't want to be buying something that won't do what I want.

Many thanks

Peter.

If the bore length is going to be 45mm I will hazard a guess that the bore diameter is a bit more than the 8mm shank of your current boring bar.

So...it will probably be ok for ruffing out, but ideally you want the largest diameter boring bar you can fit down the almost on size bore. Reducing flex and deflection.

You could knock up a boring bar from a suitable piece of bar and a ground up broken centre drill if so inclined.

John

I'm still a beginner too (bigger lathe) and the amount of holes Ilve bored is still on a one hand count. Even with spring passes I found that my 16mm boring bar still flexed enough to make a tiny difference on a 90mm bore.. which didn't matter that time but I promised myself that now I own one commercial indexable bar I'll have a go at making a larger diameter one myself for next time using the reference angles for the insert from the first. Mine is OK from an 18mm hole but the 29mm I finished with would likely have been better swapping to say a 25mm bar at the end. Of course that will mean making a holder to take it too...

Thanks John,

I have just bought a few pieces of 10mm tool steel with a view to getting a grinder and have considered after a bit of practising make ordinary tools whether I could make a more robust boring bar.

The diameter of the bore for the horizontal I'm making is 16mm and the wobbler I want to build maybe about 30mm so I don't evisage any problem getting a decent sized bar in so long as I can achieve the depth.

Peter.

Hi PGK,

What sort of 16mm bar have you ? When you say a 90mm bore do you mean depth as that would be ideal for my needs.

Edited By Peter Nichols on 29/08/2015 10:49:36

Edited By Peter Nichols on 29/08/2015 10:49:52

As a very rough guide it's generally reckoned that a normal boring bar will manage a hole 5 times the dia. of the bar in depth. Just keep making a bar to suit what you need. After a year or so you will have a good assortment

Peter,

If you have [or can contrive] the facility to attach the casting to the cross-sliide [or even perhaps the bare saddle] of the lathe ... packed so that the cylinder axis is on the centre line: Then a 'between centres boring bar' might be much more satisfactory.

MichaelG.

On the smaller lathes the tipped boring bars can get pushed away from the cut, Your potty mill engine uses an aluminium cylinder so changing to a suitable **GT tip will help.

But its quite easy to make a boring bar, for your 16mm hole I would take a piece of 12mm round mild steel, drill a hole across the end to take an old ctr drill or piece of 3mm - 1/8" HSS tool steel and then drill and tap the end for a M3 screw to retain the toolbit. Something like the middle one below

You can just make them as big as the job needs although it can be hard to get them in a standard toolpost like this 25mm one of mine!

Edited By JasonB on 29/08/2015 12:34:36

I bought my boring bar here **LINK**

But only because I wanted to play/try some indexable tooling and was unsure about the mounting angles and suitable inserts. The hole was a through hole 90mm deep 29mm wide in aluminium (and a couple of cast iron bushings to fit with a 24mm bore)

Hi all,

Many thanks for all the replies, there's a lot of helpful info there for me to look in to.

I had thought of trying to make a rudimentary one from the HSS blanks I have but never even thought of making one from a mild steel rod and piece of tool steel - funnily enough I snapped a small centre drill a few days ago and decided to keep it in case it came in handy later.

Peter.

I have a boring bar that's just a lump of 3/4" hex with a 1/8" hole across the end for a bit of HSS and tapped for an M4 screw.

I use it to bore things on the saddle and hold it in a 3-jaw, but the basic principle is so simple.

Neil

Hi,

I've just ordered a grinder yesterday, I already have some HSS blanks that I want to experiment with making 'ordinary' cutting tools but I intend to have a bash at making a boring bar as well.

I have some more materials on order for the 'Potty' mill engine I'm building so I really want to get properly started on that and need the tooling necessary to bore the cylinder.

Peter.

Hi,

Revisiting an old question?

Is it better to bore out the cylinder with a boring bar and risk it being tapered, if the lathe is not set up to turn perfectly parallel, or maybe is showing its age?

Or fit up the work piece on the carriage and use a boring bar mounted between centres; a stiffer setup and less risk of a taper but the fixed work piece setup must be spot on? It also takes more time to set it up. and you may need to make up a special boring bar to do the job.

Using the second method the diameter of the bored hole will be the same at any point throughout its length, as the tool rotates at he same radius all the time, the centres do not move, although the bored hole may not be perfectly straight because the path of the carriage that carries the work may not be perfect, the bored hole will reflect any error.

For longer bored holes I prefer the second method.

Regards
John

Edited By John McNamara on 31/08/2015 14:40:18

I agree, my father always taught me that revolving the tool rather than the work will give you a better chance of a truly round and parallel bore.

Phil

Hi all,

Thanks for the advice.

My grinder arrived today so I've managed to knock up a primitive looking boring bar using an old drill bit and some steel rod.

I've never done this before so wasn't sure about the angles etc. I'll see how it works tomorrow at least now I have a grinder I'm in a position to experiment till I can get something that works.

Peter.

Edited By Peter Nichols on 03/09/2015 17:51:38

Peter, the front edge of the cutting tool is OK but try and make the back a lot closer to the lathes axis, if you look at the lower one in my photo above you will see its almost inline with the bar/axis. This will give a better cut as what you have at the moment is not too far removed from a threading tool.

Something like this wil do a good job

Edited By JasonB on 03/09/2015 18:19:36

Hi Jason,

Thanks for that, I can see what you mean and will have another go at it tomorrow.

I did a Google search but although I found a lot about general tool grinding I couldn't find out much concerning boring bar angles.

I'm hoping to try and have a go making the cylinder for the Potty mill engine tomorrow.

Peter.

Angles are similar to an knife tool, but you need to relieve the front of the tool more. I use a curved shape to maximise the support for the cutting edge.

Neil

In response to Peter's original post I seem to remember that when I bought a shop made indexable tipped boring bar with an 8mm shank diameter I was advised by the manufacturer to expect to be able to bore about two and a half times the minimum boring diameter deep before I should expect vibration trouble. I expect that the material being bored, the speed and the condition of the lathe would change this. Locking up whatever slides are not being used might help too.

38mm deep could be about right for an 8mm shanked boring bar mounted in a tool post.

Peter,

That holder will certainly do the job

You might be disappointed by the tool bit though. The shanks of drills are often quite soft. A broken centre drill or end mill might be a better choice.

HTH

Rod